Condensation of pure refrigerants R12, R134a and their mixtures on a horizontal tube with capillary structure: An experimental study

Abstract

We experimentally studied free convection condensation heat transfer of pure refrigerants R12, R134a, and their mixtures on a horizontal single tube. Approximately equimolar mixtures of these refrigerants are azeotropic. The outside surface of the tube used had a capillary structure. The tube was integrated in an experimental set-up in a way that allowed its rotation around the axis. Movable thermocouptes inserted in the tube wall enabled the determination of the average surface temperature. This temperature, the vapour bulk temperature, and the heat flux obtained from condensate collection served for the determination of the heat transfer coefficient.The condensation heat transfer of the pure refrigerants examined is observed to change with the driving temperature difference largely in accordance with the Nusselt theory. The experimental values of the heat transfer coefficient on the tube used, however, are by a factor of 2 larger than those on a smooth tube according to this theory. Under comparable conditions, the refrigerant R134a shows by 10 to 15% better heat transfer than R12. The heat transfer of mixtures decisively depends on the compositions of their phases. Basically, the stronger the compositions of the phases differ from each other, the lower the heat transfer coefficients; they always lie below those of R134a. In the range of low temperature difference, the heat transfer coefficient of mixtures increases with the temperature difference. This is the region of the so-called partial condensation. At a larger temperature difference, a local total condensation of the mixtures takes place and the heat transfer qualitatively follows the Nusselt theory.